Followed a large class A and noticed something flopping around under his front engine compartment when he turned. Tried flashing the lights but he did not stop. Since we were on a very slow 2 lane highway, I passed him, sped up and then put my four way flashers on and stopped. I got out of the car and walked back to where he would stop. He slowly pulled up as I was pointing under his rig. He opened his window and I told him something was loose. When he figured out I had good intentions, he hopped out and found his auxiliary air hose had come unclipped. You got to love country roads where you can stop in the middle of the highway.